The present invention relates generally to gas turbine engines, and, more specifically, to rotors therein.
A typical gas turbine engine includes a compressor that compresses air which is mixed with fuel and ignited in a combustor for generating combustion gases which flow downstream through one or more turbine stages which extract energy therefrom. The turbine stages drive the compressor, and in one example, also drive an upstream fan for developing propulsion forces for powering an aircraft in flight. In this arrangement, a high pressure turbine powers the compressor and a low pressure turbine powers the fan.
Each of the fan, compressor, and turbine sections includes one or more rows or stages of rotor disks each having a plurality of circumferentially spaced apart rotor blades extending outwardly therefrom. The rotor blades typically cooperate with stationary stator vanes disposed axially adjacent thereto for suitably channeling air or combustion gases in a conventional manner.
The rotor disks which support the rotor blades have various configurations including for example a radially outer rim to which the rotor blades are suitably attached, a radially inner hub, and an annular web extending integrally therebetween. The outer diameter of the rim is selected for positioning the corresponding rotor blades at a suitable radial elevation for maximizing performance of the engine. Since the rotor disks and blades thereon typically operate at relatively high rotational speeds, the centrifugal loads generated therefrom create substantial centrifugal stress in the rotor disks which must be maintained suitably below the yield strength of the disk material for ensuring an effective useful life of the disk during operation. The centrifugal loads generated in the material being rotated are proportional to the radius from the disk centerline. The substantial centrifugal loads developed by the rotor blades must be carried by the rotor disk positioned radially inwardly thereof. The rotor disk itself also develops centrifugal loads, with the disk being the sole structural element for carrying all of the developed centrifugal loads.
Accordingly, in order to maintain centrifugal stress in the rotor disk suitably below the yield strength of the material, the disk rim, web, and hub are suitably configured for providing sufficient material for distributing the centrifugal loads to reduce the maximum developed centrifugal stress. The centrifugal loads are primarily carried in the circumferential hoop direction of the disk and generate corresponding tensile hoop stresses therein during operation. The disk web is typically axially thinner or narrower than the rim and hub for minimizing weight while suitably spreading the centrifugal loads. The hub typically includes a central bore, with the inner diameter of the hub being selected for ensuring effective distribution of the centrifugal loads for reducing centrifugal stress.
However, since the disk rotates during operation, it produces centrifugal force which must also be accommodated within the disk itself in addition to the substantial centrifugal force generated by the rotor blades attached thereto. In an aircraft gas turbine engine, for example, reducing overall engine weight is a significant design concern, with it being desirable to have rotor disks being as light as possible which is achieved by making the disk axially thin and with a relatively large hub inner diameter. Weight reduction is limited since the rotor disk must be suitably thick and the hub inner diameter must be suitably small so that the rotor disk contains sufficient material for carrying the resulting centrifugal loads.
Furthermore, as indicated above, rotors are typically found in adjoining stages and must therefore be structurally interconnected by integral extensions which define rotor shafts. The extensions may be directly bolted together, or may abut together in a simple rabbet joint or a more complex toothed joint conventionally known as a curvic coupling, with the joint being conventionally held together by a conventional axial compression load. Since the disk extensions rotate with the disks themselves, they too also add to the centrifugal loads generated during operation which must also be accommodated by the corresponding rotor disks. The shaft extensions are preferably disposed at about the same radius as the adjoining disk rims and provide an annular land with which a suitable seal may be formed with a corresponding stage of stator vanes disposed axially between adjacent stages of rotor blades.
However, it is possible for a particular rotor design, including a relatively large rim diameter and relatively high rotational speed, that the shaft extensions themselves would exceed their own yield strength making the design impractical. In such a situation, it is conventionally known to lower the radius of the shaft extensions below the radius of the corresponding disk rims, and then provide a suitable radially inner shroud for the interstage stator vanes to occupy the resulting cavity created thereby. In this way, the shaft extensions are lowered in radius to a position wherein the circumferential stress developed therein will be suitably below the yield strength of the material. However, this shrouded stator design is relatively complex and expensive to implement.